1. Field of the Invention
This invention relates to methods and compositions for removal or inactivation of hydrogen sulfide or soluble sulfide ions from drilling fluids used in drilling and completing wells in subterranean formations, and most particularly packer fluids. The advantages of the invention are particularly appreciated in high temperature, high pressure wells where the drilling fluids are oil based.
2. Description of Relevant Art
Drilling a well in a hydrocarbon bearing subterranean formation for the production of hydrocarbons from said formation typically involves use of a drilling apparatus and drilling fluid. The drilling apparatus usually comprises a bit mounted on a string of hollow steel pipe. This hollow pipe is often used to rotate the bit to enable the bit to cut into the formation. The hollow pipe also acts as a conduit for the drilling fluid to be pumped down to the bottom of the hole, from where it rises to the surface via the annulus between the drill string and the borehole wall. The drilling fluid has many functions, one of the most important of which is to convey the cuttings from the bit downhole up to the surface of the well.
Good shear thinning rheology is an important property for drilling fluids. Historically, this property was generally achieved in one of two ways with aqueous based fluids: by using a dispersion of colloidal clay minerals such as smectite clays, e.g., bentonite; or by using polymers such as xanthan gum or scleroglucan dispersed in aqueous solutions to give shear thinning fluids. Each approach has known advantages and disadvantages, although use of polymers has increasingly become preferred over clays. Systems based on dispersed colloids are susceptible to the effects of dispersing and flocculating agents, whereas the rheology of polymer based fluids is more easily controlled. A disadvantage of many polymers, however, is that, since they are polyols, they are susceptible to crosslinking at high pH by the presence of even a low concentration of multivalent cation, such as iron (III) ions.
Specialized drilling fluids called packer fluids used in drilling are typically left in the annular region of a well between the tubing and outer casing above a packer. The main functions of a packer fluid are: (1) to provide hydrostatic pressure so as to lower the differential pressure across the sealing element; (2) to lower differential pressure on the wellbore and casing to prevent collapse; and (3) to protect metals and elastomers from corrosion. In well completions, packer fluids remain in the well tubing-casing annulus above the packer after the well completion has been run and all circulation devices have been isolated. While packer fluids are prepared for the requirements of the given completion, they generally should be of sufficient density to control the producing-formation, be solids-free and resistant to viscosity changes over long periods of time, and be noncorrosive to the wellbore and completion components.
In drilling some subterranean formations, and often particularly those bearing oil or gas, hydrogen sulfide accumulations are frequently encountered. The drilling fluid brings the hydrogen sulfide to the surface. Such sulfide in the drilling fluid is problematic, as it can corrode the steel in the drilling apparatus and may be liberated into the atmosphere as toxic sulfide gas at the well surface.
Generally, to protect the health of those working with the drilling fluid and those at the surface of the well, conditions should be maintained to ensure that the concentration of hydrogen sulfide above the fluid, emitted due to the partial pressure of the gas, is less than about 15 ppm. The partial pressure of hydrogen sulfide at ambient temperature is a function of the concentration of sulfide ions in the fluid and the pH of the fluid. To ensure that the limit of 15 ppm is not exceeded even for the maximum sulfide concentration that may be encountered in a subterranean formation, the pH of the drilling fluid is typically maintained at a minimum of about 11.5. Also, to prevent the soluble sulfide concentration in the fluid from becoming excessive, action is routinely taken to remove sulfide from the fluid.
A common process for removing sulfide from drilling fluids is by precipitation, usually with a solid zinc compound. Zinc compounds commonly used are zinc oxide and zinc carbonate. These compounds react with hydrogen sulfide to form insoluble zinc sulfide. In insoluble form, the sulfide is relatively harmless (unless the pH falls to acid conditions) and can be removed from the fluid by known separation techniques.
Because these zinc compounds are solids, the reaction rate can be slow, which is especially undesirable when high concentrations of sulfide are encountered or when removal of final traces of sulfide is desired. However, soluble zinc salts can not be used as they hydrolyze at low pH, forming gelatinous hydroxide. While the gelatinous hydroxide would still react with the sulfide, the gelatinous solid would likely interfere with the rheological properties of the drilling fluid. Also, zinc salts behave as acids and tend to reduce the pH of the fluid, increasing the risk of greater emission of hydrogen sulfide into the air.
U.S. Pat. No. 4,252,655 to Carney discloses a soluble form of zinc in a chelated form for use as a hydrogen sulfide scavenger in an oil well. The chelating agents disclosed in U.S. Pat. No. 4,252,655 are low molecular weight hydrocarbon based materials containing acetic or nitrogen functional groups with a stability constant in the range of about 10-16, as described by Chaber Martell in Organic Sequestering Agents. Zinc chelates with a stabilizing constant outside of this range of about 10-16 are said to be either ineffective for removing soluble sulfide ion or to adversely affect the rheology of the well fluid.
Various zinc compounds—both soluble and insoluble—have been used as sulfide scavengers for many years and historically were generally regarded as an industry standard. However, all zinc compounds have the disadvantage that zinc is regarded as a toxic heavy metal whose discharge must be carefully controlled to protect the environment. When a zinc scavenger is used in the drilling fluid, the cuttings, as well as the residual fluid at the end of the drilling operation, will be contaminated with zinc. Environmental awareness has increasingly made use of zinc unacceptable.
Most other heavy metals which react with hydrogen sulfide to form insoluble sulfide such as copper (both oxidation states), mercury, lead, and nickel, also cause environmental concerns and thus are no more acceptable than zinc.
An exception is iron, a metal, that also forms insoluble sulfide upon reaction with hydrogen sulfide but which is largely free from environmental concerns. Iron salts, however, have been found to be unsuitable as sulfide scavengers because the compounds are not stable in solutions at high pH. At the pH used in drilling fluids, both iron (II) and iron (III) are precipitated as gelatinous iron hydroxide, which would have unacceptable effects on the rheology of the drilling fluid.
U.S. Pat. No. 4,756,836 to Jeffrey et al. teaches using an iron chelate as a downhole hydrogen sulfide scavenger in drilling mud, particularly water based clay muds. This patent discloses chelates of iron with hydroxethylethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA). The chelates are said to convert hydrogen sulfide to sulphur. Claimed advantages of this invention are said to be that the iron chelate is regenerated by oxygen at the surface and that the iron scavenges oxygen in the mud stream to cut down oxygen assisted corrosion of the drill stem. This patent further teaches that whether the iron is supplied in the Fe (II) or Fe(III) form, exposure to oxygen at some point in the mud flow changes the form to Fe (III) to prepare the chelate for hydrogen sulfide conversion. Oxygen exposure in an aerated mud pit or in the shale shaker or by another oxygen source is said to aid regeneration of the iron chelate.
While iron (III) is known to readily chelate with EDTA, NTA and HEDTA and DTPA, such complexes have limited stability at high pH. Iron in these complexes is well known to tend to precipitate out as ferric hydroxide at a pH greater than 9. For example, manufacturers of these chelates typically quote stability or effectiveness as an Fe(III) chelate, of NTA at pH 1-3, DPTA at pH 1-7, EDTA at pH 1-6, and HEDTA at pH 1-9. At pH higher than such ranges, these chelating agents lack ability to stabilize the iron against precipitation as the hydroxide. For effective use as a scavenger according to the teachings of Jeffrey's U.S. Pat. No. 4,756,836, the iron must stay in chelated form. Further, the multivalent nature of iron III is likely to cause crosslinking of polymers in a polymer based drilling mud, leading to gelation and interference with the rheology of the fluid.
U.S. Pat. No. 6,746,611 to Davidson provides an environmentally friendly method of removing hydrogen sulfide or sulfide ions from fluids having a pH in excess of about 9 where ferrous gluconates, as iron chelating agents, are added to the fluid in quantities sufficient to form iron sulfide with the sulfide ion. This patent also teaches an additive for drilling fluids comprising iron chelating agents, particularly ferrous gluconate, in an effective amount for removing hydrogen sulfide from the fluid. The additive has stability at a pH in excess of about 9 and as high as about 11 to 12 or higher.
Iron gluconate is now recognized as a desirable non-toxic hydrogen sulfide scavenger, effective in aqueous drilling fluids, and it has also been successfully used in some oil based drilling fluids, including invert emulsion based drilling fluids. Oil based drilling fluids, typically comprising mineral oils or synthetic oils, are most commonly used in high temperature, high pressure wells because the temperatures are too high for aqueous based fluids. Those wells also commonly have hydrogen sulfide present. A continuing need exists for environmentally friendly hydrogen sulfide scavengers suitable and effective for use in drilling fluids.